Cytogenetic analyses of 221 patients in clinical center protocols were reported. Clinical studies of several endocrine disorders required confirmation of cytogenetic diagnosis and detection of genetic variation possibly affecting therapeutic response. Cytogenetic screening was also done to determine protocol eligibility or to reveal constitutional aberrations which might reveal gene localization in unmapped single gene disorders. In some diseases, we searched for secondary aberrations which might play a role in development of clinical abnormalities. No cytogenetic abnormalities were seen in 29 new patients with mental disorders. However, a translocation previously detected in a schizophrenic patient was found in his relatives; cell lines were established from the family but linkage studies were deferred because the rearrangement and mental problems were not consistently associated. Fluorescent in-situ hybridization (FISH) with chromosome-specific satellite DNA probes was used for investigation of a patient with premature ovarian failure and an X-autosome rearrangement, twin sisters with the Turner syndrome and a minute Y chromosome, a previously reported patient with mosaic Down syndrome, and a 46, XX male with hermaphroditism. Cell lines from several of these patients are being studied to detect and localize molecular defects and correlate them with clinical manifestations. The unit is affiliated with the NIH medical genetics program, is accredited by the ABMG to provide post-doctoral training in clinical laboratory cytogenetics, and provides clinical consultations. It also participates in the College of American Pathologists cytogenetics proficiency testing program, which is required for accreditation of the genetics and pathology training programs.